Essence Magic is one of the two Dark Ones, (immoral forms of witchcraft) allowing the user to consume the power of a witch using their heart, after their death, which strengthens their own abilities to a high magnitude. Essence Magics gives the user an endless supply of strength because the witch is dead. This magic requires the heart of the person, a spell, and ritual, taking great strength to perform successfully. The more witches or the more powerful the witch the user feeds on, the more strength they earn.
Dark witches perform this to gain more powers to stand against their enemies. Taking in enough can even help the user gain advanced Earth Magic.
While the supply is endless, the Bonum Coven was once able to momentarily weaken the supply of Matthew's Essence Magic with the help of Isabel's spell.